Slew of challenges for Republican-led US Congress in 2015

The 113th Congress, which wrapped up December 16, was one of the least productive in modern history. Onward to 2015.

US lawmakers return to Washington on Tuesday under a new, fully Republican banner after the party swept to victory in November elections.

Republican leaders will have plenty on their plate in the first quarter, as the party seeks to roll back President Barack Obama's legislative gains and press its own agenda ahead of the 2016 presidential race.

With a battle brewing, the government can use so-called extraordinary measures to stave off default for a few months.

- Foreign policy -

The Senate will swiftly address Iran's nuclear program, according to Republican Senator Mark Kirk, who co-authored new sanctions on Iran.

The White House has warned Congress that tightening sanctions could destroy international negotiations aimed at preventing Iran from developing an atomic bomb.

But with bipartisan support for a firm stance, Obama could potentially face a tough call on whether to veto legislation that would impose sanctions and require Congress to sign off on any final deal.

Lawmakers will also debate a prospective new authorization for use of military force against Islamic State extremists.

The administration conducts its current operations -- including more than 1,100 airstrikes in Syria and Iraq since September targeting IS -- under authority passed in 2001 and 2002 after Al-Qaeda's 9/11 attacks on the United States.

Many Democrats have bucked Obama and demanded a limited, three-year authority that prohibits the use of US ground troops.

- Obamacare -

Since the Affordable Care Act became law in 2010, House Republicans have voted more than 50 times to repeal or defund it, mostly to no avail, as the Democratically-run Senate refused to take up the measures.

That changes with Republicans in charge, and in an op-ed after November's elections McConnell and House Speaker John Boehner said they were "renewing our commitment to repeal Obamacare."

McConnell has acknowledged a full repeal is not possible, and is seeking to limit the law's scope.

He signaled Republicans will seek to redefine the work week to 40 hours, up from Obamacare's current 30-hour threshold.